I think people are pushing for Mysterio as it would make the most sense with how the last film ended. Webb has already said TASM3 will focus on PP's recovery, naturally you want a villian that can hinder that recovery, it makes the villians motivations/the plot feel less contrived as the goals of the antag/protag intertwine and contradict each other, you get natural tension, and something that doesn't feel as forced as say, youuuuuu'rrreeeeee a frraaaauuuud
Now I see Webb as a director that does the more human stuff better, which is why a villian that you could potentially steer towards more of a psychological conflict seems appealing to me.
Plus for the Sony execs, he has great visual potential, so they can still sell the film visually, Kraven would be hard to sell in a trailer, and some might see it as a Tarzan vs. Spider-Man spinoff to compete with Superman vs. Batman.